In radio communication systems, signals propagate from the transmitter to the receiver in the form of electromagnetic waves in a radio channel. A signal transmitted in the radio channel is affected by different kinds of distortions and therefore the received signal deviates from the one transmitted. Despite the distortion, the transmitted information has to be detected in the receiver. Various methods have been developed to ensure successful signal detection, for example search methods, or algorithms, based on diverse diagrams, such as trellis diagrams or tree diagrams. These algorithms are used in an attempt to find the path, i.e. symbol sequence (a symbol sequence comprising at least one bit), leading through the diagram in which the path metric is the smallest, the path metric being typically the sum of the Hamming weights or the squared Euclidean distances of different state transitions; in other words, they are based on the Viterbi algorithm. The Viterbi algorithm is extremely complex and therefore slow, which is why it is usually not suitable as such for practical applications. The algorithms referred to are suboptimal: they are many and they are applicable to different purposes of use. They are used for example in channel equalizers, for decoding signals coded in various ways, in speech recognition, and in multi-user detectors (MUD). Examples of the algorithms include M-algorithm, Fano algorithm, decision-feedback sequence estimation (DFSE), which are all hard decision algorithms. Soft decision algorithms include soft decision DFSE and a soft decision M-algorithm.
However, if the first value of a channel impulse response is not the highest value, prior art algorithms function deficiently. In previous attempts to solve this problem, a prefilter is arranged in front of the channel equalizer to concentrate the received signal's energy at the beginning of the impulse response. The prefilter, however, increases the complexity of the system and lengthens the time needed for processing the signal. There are situations where even the prefilter does not manage to provide enough power for the first impulse response value. In addition, if the first impulse response value used for determining the channel impulse response is unreliable or too low, the reliability of sequence estimation is impaired.